Tunable rare-earth fcu-metal-organic frameworks

ABSTRACT

Embodiments of the present disclosure describe compositions comprising a molecular building block (MBB) having the formula [RE6(μ3-OH)8(O2C—)6N4C—)6] or [RE6(μ3-OH)8(O2C—)12], wherein RE is a rare earth metal ion. Embodiments of the present disclosure further describe methods of making a metal-organic framework (MOF) composition comprising contacting one or more molecular building blocks (MBBs) with one or more bidentate ligands, wherein the MBBs have the formula [RE6(μ3-OH)8(O2C—)6N4C—)6] or [RE6(μ3-OH)8(O2C—)12], wherein RE is a rare earth metal ion.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of Ser. No. 15/926,511, filed on Mar. 20, 2018, which is a Continuation of Ser. No. 14/919,238, filed on Oct. 21, 2015, which issued a U.S. Pat. No. 9,920,076 on Mar. 20, 2018, which is a Continuation of Ser. No. 14/019,511, filed on 5 Sep. 2013, which issued as U.S. Pat. No. 9,266,907 on Feb. 23, 2016 and which application is incorporated herein by reference. A claim of priority is made.

TECHNICAL FIELD

This invention relates to metal-organic frameworks having tunable structures.

BACKGROUND

Metal-organic framework (MOF) materials can have tunable properties based on their structure, including porosity. Unique porous structures can allow the material to be used in applications including gas sequestration, storage and separation or scrubbing.

SUMMARY

In general, embodiments of the present disclosure describe metal-organic frameworks comprising rare earth metal ions and bidentate ligands, wherein the metal-organic frameworks have a face-centered cubic topology.

Accordingly, embodiments of the present disclosure describe a composition comprising a molecular building block (MBB) having the formula [RE₆(μ₃-OH)₈(O₂C—)₆N₄C—)₆], wherein RE is a rare earth metal ion.

Embodiments of the present disclosure describe a composition comprising a molecular building block (MBB) having the formula [RE₆(μ₃-OH)₈(O₂C—)₁₂], wherein RE is a rare earth metal ion.

Embodiments of the present disclosure describe a method of making a metal-organic framework (MOF) composition comprising contacting one or more molecular building blocks (MBBs) with one or more bidentate ligands, wherein the MBBs have the formula [RE₆(μ₃-OH)₈(O₂C—)₆N₄C—)₆] or [RE₆(μ₃-OH)₈(O₂C—)₁₂], wherein RE is a rare earth metal ion.

Other aspects, embodiments, and features will be apparent from the following description, the drawings, and the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing representing ball-and-stick and schematic representation of 1: From top to bottom, organic and inorganic MBBs, FTZB²⁻ and the 12-connected Tb-based cluster, respectively, which can be viewed as a linear connection and cuboctahedron node to afford the augmented fcu net, consisting of octahedral and tetrahedral cages shown as blue and pink truncated polyhedron, respectively, Hydrogen atoms and coordinated water molecules are omitted for clarity. Tb=green, C=gray, N=blue, O=red, F=purple.

FIG. 2 are graphs representing PXRD patterns for compound 1: (a) after exposure to water and (b) variable temperature under a vacuum.

FIG. 3 are graphs representing (a) CO₂ data for 1 and 2 at 298 K and (b)

_(st) in 1 and 2 for CO₂ calculated from the 258, 273 and 298 K isotherms.

FIG. 4 are graphs representing (a)

_(st) for CO₂ of compound 1 in sites I, II, and III compared to the total

_(st) as determined by the TSL model and CO₂ adsorption isotherms of compound 1 for sites I (b), II (c), and III (d) using the TSL model.

FIG. 5 are graphs representing (a) CO₂ selectivity over N₂ resulted from the interaction with site I at 298 K at different total pressures in 0.5-2.0 bar range calculated using IAST for compound 1 and (b) experimental breakthrough test of traces (1000 ppm) CO₂ in mixture with N₂ on compound 1.

FIGS. 6A and 6B are graphs representing PXRD patterns of the as-synthesized, calculated and solvent-exchanged compounds 1-2, indicating the phase purity of as-synthesized and methanol-exchanged products.

FIG. 7 is a graph representing PXRD patterns of the as-synthesized, calculated and solvent-exchanged compound 3, indicating the phase purity of as-synthesized and methanol-exchanged products.

FIGS. 8A and 8B are graphs representing PXRD patterns of the as-synthesized, calculated and solvent-exchanged compounds 4-5, indicating the phase purity of as-synthesized and solvent-exchanged products.

FIGS. 9A and 9B are graphs representing PXRD patterns of the as-synthesized, calculated and solvent-exchanged compounds 6-7, indicating the phase purity of as-synthesized and solvent-exchanged products.

FIG. 10 is a graph representing PXRD patterns of the as-synthesized compounds 1 and 2 compared with the La, Eu and Yb fcu-MOF analogs.

FIG. 11 is a graph representing PXRD patterns for compound 2 after exposure to water, indicating a highly chemical stability in aqueous media.

FIGS. 12A and 12B are graphs representing TGA plots of the as-synthesized and methanol-exchanged compounds 1-2.

FIG. 13 are graphs representing TGA plots of the as-synthesized and methanol-exchanged compound 3.

FIGS. 14A and 14B are graphs representing TGA plots of the as-synthesized and solvent-exchanged compounds 4-5.

FIGS. 15A and 15B are graphs representing TGA plots of the as-synthesized and solvent-exchanged compounds 6-7.

FIG. 16 is a graph representing variable-temperature (VT) PXRD of compound 2, revealing the thermal stability up to 275 degree C.

FIG. 17 is a ball-and-stick representation of compound 1, constructed from the assembly of 12-connected carboxylate/tetrazolate-based molecular building blocks (MBBs) linked together via a linear and heterofunctional FTZB organic linker, to give a 3-periodic fcu-MOF with two types of polyhedral cages: i.e. tetrahedral and octahedral.

FIG. 18 is a ball-and-stick representation of compound 6, constructed from the assembly of 12-connected carboxylate-based MBBs linked together via a ditopic FBPDC organic linker, to give a 3-periodic fcu-MOF with two types of polyhedral cages.

FIG. 19 is a synergetic effect representation of a CO₂ surrounded by an open metal site, uncoordinated nitrogen atoms of tetrazolate and polarizable fluoro atom as well as hydroxo moieties.

FIG. 20 are graphs representing Ar sorption isotherms collected at 87 K (a), pore size distribution analysis (b) for compound 1.

FIG. 21 are graphs representing H₂ sorption data for compound 1: (a) fully reversible H₂ isotherms collected at 77 and 87 K and (b)

_(st) for H₂ calculated from the corresponding isotherms.

FIG. 22 are graphs representing CO₂ sorption data for compound 1: (a) fully reversible VT CO₂ isotherms and (b)

_(st) for CO₂ calculated from the corresponding isotherms FIG. 23 are graphs representing Ar sorption isotherms collected at 87 K (a), pore size distribution analysis (b) for compound 2.

FIG. 24 are graphs representing H₂ sorption data for data for compound 2: (a) fully reversible H₂ isotherms collected at 77 and 87 K and (b)

_(st) for H₂ calculated from the corresponding isotherms.

FIG. 25 are graphs representing CO₂ sorption data for compound 2: (a) fully reversible VT CO₂ isotherms and (b)

_(st) for CO₂ calculated from the corresponding isotherms.

FIG. 26 are graphs representing CO₂ adsorption isosters for compounds 1 (a) and 2 (b), confirming the accuracy of the

_(st) determined from VT CO₂ adsorption isotherms as evidenced by the linearity in the isosters.

FIG. 27 is a graph representing

_(st) for CO₂ of compound 2 in sites I, II and III compared to the total

_(st) as determined by the TSL model.

FIGS. 28A-28C are graphs representing CO₂ adsorption isotherms of compound 2 for sites I, II and III using the TSL model.

FIG. 29 are graphs representing Ar sorption isotherms collected at 87 K (a), pore size distribution analysis (b) for compound 3.

FIG. 30 are graphs representing H₂ sorption data for data for compound 3: (a) fully reversible H₂ isotherms collected at 77 and 87 K and (b)

_(st) for H₂ calculated from the corresponding isotherms.

FIG. 31 are graphs representing CO₂ sorption data for compound 3: (a) fully reversible VT CO₂ isotherms and (b)

_(st) for CO₂ calculated from the corresponding isotherms.

FIG. 32 is a graph representing ¹⁹F NMR spectrum of compound 3 digested in HCl and DMSO, showing the presence of the modulator, 2-fluorobenzoic acid, and thus resulting in a reduced pore volume compared to the theoretical SCXRD data (i.e. 0.39 vs 0.55 cm³ g⁻¹).

FIG. 33 are graphs representing Ar sorption isotherms collected at 87 K (a), pore size distribution analysis (b) for compound 4.

FIG. 34 are graphs representing H₂ sorption data for compound 4: (a) fully reversible H₂ isotherms collected at 77 and 87 K and (b)

_(st) for H₂ calculated from the corresponding isotherms.

FIG. 35 are graphs representing CO₂ sorption data for compound 4: (a) fully reversible VT CO₂ isotherms and (b)

_(st) for CO₂ calculated from the corresponding isotherms.

FIG. 36 are graphs representing Ar sorption isotherms collected at 87 K (a) and pore size distribution analysis (b) for compound 5.

FIG. 37 are graphs representing H₂ sorption data for compound 5: (a) fully reversible H₂ isotherms collected at 77 and 87 K and (b)

_(st) for H₂ calculated from the corresponding isotherms.

FIG. 38 are graphs representing CO₂ sorption data for compound 5: (a) fully reversible VT CO₂ isotherms and (b)

_(st) for CO₂ calculated from the corresponding isotherms

FIG. 39 are graphs representing Ar sorption isotherms collected at 87 K (a), pore size distribution analysis (b) for compound 6.

FIG. 40 are graphs representing H₂ sorption data for compound 6: (a) fully reversible H₂ isotherms collected at 77 and 87 K and (b)

_(st) for H₂ calculated from the corresponding isotherms.

FIG. 41 are graphs representing CO₂ sorption data for compound 6: (a) fully reversible VT CO₂ isotherms and (b)

_(st) for CO₂ calculated from the corresponding isotherms.

FIG. 42 are graphs representing Ar sorption isotherms collected at 87 K (a), pore size distribution analysis (b) for compound 7.

FIG. 43 are graphs representing H₂ sorption data for compound 7: (a) fully reversible H₂ isotherms collected at 77 and 87 K and (b)

_(st) for H₂ calculated from the corresponding isotherms.

FIG. 44 are graphs representing CO₂ sorption data for compound 7: (a) fully reversible VT CO₂ isotherms and (b)

_(st) for CO₂ calculated from the corresponding isotherms.

FIG. 45 are graphs representing CO₂ adsorption isosters for compounds 4 (a) and 5 (b).

FIG. 46 are graphs representing CO₂ adsorption isosters for compounds 3 (a) and 6 (b).

FIG. 47 are graphs representing

_(st) for CO₂ of compounds 3 (a) and 6 (b) in sites I and II compared to the total

_(st) as determined by the DSL model.

FIG. 48 are graphs representing CO₂ adsorption isotherms of compound 3 for sites I and II using the DSL model.

FIG. 49 are graphs representing CO₂ adsorption isotherms of compound 6 for sites I and II using the DSL model.

FIG. 50 is a schematic diagram representing the Rubotherm gravimetric-densimetric apparatus.

FIGS. 51A-51F are graphs representing excess high-pressure sorption isotherms for compound 1: H₂, CO₂, CH₄, N₂ and O₂. The adsorption and desorption branches are represented as solid and open symbols, respectively.

FIG. 52A-52F are graphs representing excess high-pressure sorption isotherms for compound 2: H₂, CO₂, CH₄, N₂ and O₂. The adsorption and desorption branches are represented as solid and open symbols, respectively.

FIGS. 53A and 53B are graphs representing water vapor sorption isotherms collected at 298 K for compound 1 (top) and compound 2 (bottom) with adsorption (solid symbols) and desorption (open symbols) points, showing that both materials are tolerant to water. Note that the last desorption point corresponds to the coordinated water molecules in each material, i.e., 5.50 water per Tb6 cluster and 5.76 water per Y6 cluster.

FIG. 54 is a graph representing CO₂ adsorption kinetics curve for compound 2 at 0.2 bar and 298 K (collected during adsorption measurements).

FIG. 55 are graphs representing CO₂ selectivity over N₂ resulted from the interaction with site I at 298 K at different total pressures in 0.5-2 barrange calculated using IAST for compound 1.

FIG. 56 is a graph representing experimental breakthrough test of traces (1000 ppm) CO₂ in mixture with N₂ on compound 1.

FIG. 57 is a graph representing CO₂ over N₂ selectivity for compound 1 and 2 calculated using IAST for CO₂/N₂: 10/90 gas mixture at 298 K.

FIG. 58 is a collection of images representing SEM image for compound 1 (top), showing the uniform polyhedral morphology of the crystals and the optical images for compound 3 with different sizes due to varying the ethanol concentration during synthesis (bottom).

DETAILED DESCRIPTION

A metal-organic framework composition can have a face centered cubic (fcu) structure composed of metal ions and bidentate ligands. The metal ions and bidentate ligands for molecular building blocks that further form the fcu structure.

The molecular building block (MBB) approach has recently emerged as a powerful strategy for the design and construction of solid-state materials. See, e.g., Stein et al., Science 1993, 259, 1558-1564; Ferey, G., J. Solid State Chem. 2000, 152, 37-48; Eddaoudi et al., Science 2002, 295, 469-472; Kitagawa et al., Angew. Chem. Int. Ed. 2004, 43, 2334-2375; Moulton et al., Chem. Rev. 2001, 101, 1629-1658; Eddaoudi et al., Acc. Chem. Res. 2001, 34, 319-330; and U.S. Pat. No. 6,624,318 (each of which is incorporated by reference herein in its entirety). The molecular building block joins or otherwise associates with other molecular building blocks to form supramolecular structures. The molecular building block can be a 12-connected molecular building block. The 12-connected molecular building block can have 12 sites for ligand attachment to neighboring structures.

The metal ions can form a metal ion component of the composition. The metal ion can be an electron rich metal ion. For example, the metal ion can be a RE metal ion, for example, a lanthanide elements, such as an ion of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or Y. In certain circumstances, the metal ion is terbium or yttrium, e.g., Tb⁺³ or Y⁺³.

The bidentate ligand can form a bidentate ligand component of the composition. The bidentate ligand has two anionic binding groups. The two anionic binding groups, point away from each other. Specifically, the two anionic binding groups can be oriented 180 degrees from each other. The bidentate ligand can have the structure: A1-L-A2  (I)

In formula (I), each A1 can be carboxyl, tetrazolyl, sulfonyl, or phosphoryl;

In formula (I), each A2 can be carboxyl, tetrazolyl, sulfonyl, or phosphoryl;

In preferred embodiments, A1 and A2 are each, independently, carboxyl or tetrazolyl.

In formula (I), L can be a divalent aryl, heteroaryl, carbocyclyl, or heterocyclyl, In preferred embodiments, L can be a 3- to 14 membered divalent monocyclic heterocyclyl, a 3- to 14 membered divalent aryl, or a 3- to 14 membered divalent heteroaryl. In preferred embodiments, L is substituted with 1, 2, 3, or 4 halo or halomethyl groups. For example, L can be an ortho substituted fluoro phenylene, naphthylene or diphenylene group.

The term “aryl” refers to monocyclic, bicyclic or tricyclic aromatic hydrocarbon groups having from 6 to 14 carbon atoms in the ring portion. In one embodiment, the term aryl refers to monocyclic and bicyclic aromatic hydrocarbon groups having from 6 to 10 carbon atoms. Representative examples of aryl groups include phenyl, naphthyl, fluorenyl, and anthracenyl.

The term “aryl” also refers to a bicyclic or tricyclic group in which at least one ring is aromatic and is fused to one or two non-aromatic hydrocarbon ring(s). Nonlimiting examples include tetrahydronaphthalene, dihydronaphthalenyl and indanyl.

As used herein, the term “heterocyclyl” refers to a saturated or unsaturated, non-aromatic monocyclic, bicyclic or tricyclic ring system which has from 3- to 15-ring members at least one of which is a heteroatom, and up to 10 of which may be heteroatoms, wherein the heteroatoms are independently selected from O, S and N, and wherein N and S can be optionally oxidized to various oxidation states. In one embodiment, a heterocyclyl is a 3-8-membered monocyclic. In another embodiment, a heterocyclyl is a 6-12-membered bicyclic. In yet another embodiment, a heterocyclycyl is a 10-15-membered tricyclic ring system. The heterocyclyl group can be attached at a heteroatom or a carbon atom. Heterocyclyls include fused or bridged ring systems. The term “heterocyclyl” encompasses heterocycloalkyl groups. The term “heterocycloalkyl” refers to completely saturated monocyclic, bicyclic or tricyclic heterocyclyl comprising 3-15 ring members, at least one of which is a heteroatom, and up to 10 of which may be heteroatoms, wherein the heteroatoms are independently selected from O, S and N, and wherein N and S can be optionally oxidized to various oxidation states. Examples of heterocyclyls include dihydrofuranyl, [1,3]dioxolane, 1,4-dioxane, 1,4-dithiane, piperazinyl, 1,3-dioxolane, imidazolidinyl, imidazolinyl, pyrrolidine, dihydropyran, oxathiolane, dithiolane, I,3-dioxane, 1,3-dithianyl, oxathianyl, thiomorpholinyl, oxiranyl, aziridinyl, oxetanyl, azetidinyl, tetrahydrofuranyl, pyrrolidinyl, tetrahydropyranyl, piperidinyl, morpholinyl, piperazinyl, azepinyl, oxapinyl, oxazepinyl and diazepinyl.

The term “spiroheterocycloalkyl” as used herein, is a heterocycloalkyl that has one ring atom in common with the group to which it is attached. Spiroheterocycloalkyl groups may have from 3 to 15 ring members. In a preferred embodiment, the spiroheterocycloalkyl has from 3 to 8 ring atoms selected from carbon, nitrogen, sulfur and oxygen and is monocyclic.

As used herein, the term “heteroaryl” refers to a 5-14 membered monocyclic-, bicyclic-, or tricyclic-ring system, having 1 to 10 heteroatoms independently selected from N, O or S, wherein N and S can be optionally oxidized to various oxidation states, and wherein at least one ring in the ring system is aromatic. In one embodiment, the heteroaryl is monocyclic and has 5 or 6 ring members. Examples of monocyclic heteroaryl groups include pyridyl, thienyl, furanyl, pyrrolyl, pyrazolyl, imidazoyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, triazolyl, oxadiazolyl, thiadiazolyl and tetrazolyl. In another embodiment, the heteroaryl is bicyclic and has from 8 to 10 ring members. Examples of bicyclic heteroaryl groups include indolyl, benzofuranyl, quinolyl, isoquinolyl indazolyl, indolinyl, isoindolyl, indolizinyl, benzamidazolyl, quinolinyl, 5,6,7,8-tetrahydroquinoline and 6,7-dihydro-5H-pyrrolo[3,2-d]pyrimidine.

As used herein, the term “carbocyclyl” refers to saturated or partially unsaturated (but not aromatic) monocyclic, bicyclic or tricyclic hydrocarbon groups of 3-14 carbon atoms, preferably 3-9, or more preferably 3-8 carbon atoms. Carbocyclyls include fused or bridged ring systems. The term “carbocyclyl” encompasses cycloalkyl groups. The term “cycloalkyl” refers to completely saturated monocyclic, bicyclic or tricyclic hydrocarbon groups of 3-12 carbon atoms, preferably 3-9, or more preferably 3-8 carbon atoms. Exemplary monocyclic carbocyclyl groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclopentenyl, cyclohexyl or cyclohexenyl. Exemplary bicyclic carbocyclyl groups include bornyl, decahydronaphthyl, bicyclo[2.1.1]hexyl, bicyclo[2.2.1]heptyl, bicyclo[2.2.1]heptenyl, 6,6-dimethylbicyclo[3.1.1]heptyl, 2,6,6-trimethylbicyclo[3.1.1]heptyl, or bicyclo[2.2.2]octyl. Exemplary tricyclic carbocyclyl groups include adamantyl.

As used herein, the term “halocycloalkyl” refers to a cycloalkyl, as defined herein, that is substituted by one or more halo groups as defined herein. Preferably the halocycloalkyl can be monohalocycloalkyl, dihalocycloalkyl or polyhalocycloalkyl including perhalocycloalkyl. A monohalocycloalkyl can have one iodo, bromo, chloro or fluoro substituent. Dihalocycloalkyl and polyhalocycloalkyl groups can be substituted with two or more of the same halo atoms or a combination of different halo groups.

The term “aryl” also refers to a bicyclic or tricyclic group in which at least one ring is aromatic and is fused to one or two non-aromatic hydrocarbon ring(s). Nonlimiting examples include tetrahydronaphthalene, dihydronaphthalenyl and indanyl.

The term “arylalkyl” refers to an alkyl group substituted with an aryl group. Representative examples of arylalkyl groups include, for example, benzyl, picolyl, and the like.

The term “phenylene” refers to a divalent phenyl.

The molecular building block can include bridging ligands, such as, for example, oxy, hydroxyl, sulfhydryl, or amino groups.

In the synthesis of the molecular building blocks, the molecular building block can have an overall ionic charge. Thus the molecular building block can be an anion or a cation and have one or more corresponding counterions, such as, for example, H⁺, Li⁺, Na⁺, K⁺, Mg⁺, Ca²⁺, Sr²⁺, ammonium (including monoalkyl, dialkyl, trialkyl or tetraalkylalkyl ammonium), or one or F⁻, Cl⁻, Br⁻, I⁻, ClO⁻, ClO₂ ⁻, ClO₃ ⁻, ClO₄ ⁻, OH⁻, NO₃ ⁻, NO₂ ⁻, SO₄ ²⁻, SO₃ ²⁻, PO₄ ³⁻, CO₃ ²⁻, borate (including monoalkyl, dialkyl, trialkyl or tetraalkylalkyl borate) or PF₆ ⁻, and organic counterions such as acetate or triflate.

The A1 and A2 groups are oriented at 180 degrees from each other. For example, when L is arylene, A1 and A2 are in a “para” or substantially “para” relative position. In a phenylene structure, A1 and A2 are at positions 1 and 4 on the ring; in a biphenylene structure, A1 and A2 are at positions 4 and 4′.

The method of making a MOF composition can include contacting a metal ion component with a bidentate ligand component having two anionic binding groups. A salt of the metal ion can be dissolved in a solvent and combined with the bidentate ligand. Optionally, other salts can be added to provide other counter ions in the final structure. The material is then crystallized from the combined solution. The presence of a hydrophobic group in the bidentate ligand, for example, a fluoro group ortho to the binding group, contributes to formation of the desired fcu structure. The bidentate ligand having a hydrophobic group can be present in a catalytic amount during formation of the final MOF.

A series of fcu-MOFs based on RE metals and linear fluorinated/non-fluorinated, homo-/hetero-functional ligands can be targeted and synthesized. This particular fcu-MOF platform was selected due to its unique structural characteristics combined with the ability/potential to dictate and regulate its chemical properties (e.g., tuning of the electron-rich rare-earth metal ions and high localized charge density, a property arising from the proximal positioning of polarizing tetrazolate moieties and fluoro-groups that decorate the exposed inner surfaces of the confined conical cavities). These features permitted a systematic gas sorption study to evaluate/elucidate the effects of distinctive parameters on CO₂-MOF sorption energetics. It shows the importance of the synergistic effect of exposed open metal sites and proximal highly localized charge density toward materials with enhanced CO₂ sorption energetics.

In recent years, there has been a strong scientific drive to minimize greenhouse gas emissions especially CO₂. See, for example, Chu, S. Science 2009, 325, 1599, which is incorporated by reference in its entirety. The release of CO₂ from flue gas and the automobile industry are the major contributors, and myriad efforts are underway to economically separate and capture the effluent CO₂. See, for example, The Center for Climate and Energy Solutions (C2ES), Reducing Greenhouse Gas Emissions from U.S. Transportation, 2011, Arlington; Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Chem. Rev. 2012, 112, 724-781, Vaidhyanathan, R.; Iremonger, S. S.; Shimizu, G. K. H.; Boyd, P. G.; Alavi, S.; Woo, T. K. Science 2010, 330, 650-653, each of which is incorporated by reference in its entirety. Highly porous sorbent materials have emerged as a plausible solution, and considerable efforts have been put forth to develop suitable materials. An optimal adsorbent for CO₂ separation should, in addition to high adsorption uptake and suitable kinetics, exhibit high affinity toward CO₂ to be translated into high interaction, which in turns plays a critical role in determining the adsorption selectivity and the energy required to release CO₂ during the regeneration step. Accordingly, the ideal isosteric heat of adsorption (Qs) should permit reversible physical adsorption-desorption operation in a pressure or vacuum swing adsorption (PSA or VSA) process (i.e., CO₂— sorbent interactions are neither too strong nor too weak).

MOFs, a relatively new class of porous materials, appear well-poised to address the CO₂ challenge due to their mild synthesis conditions, relatively high thermal stability, large pore volumes, potentially exposed inner surface with high localized charge density, and readily programmable and modular construction (i.e., a given structure with the desired net topology; functionalizable isoreticular structures) from pre-designed molecular building blocks (MBBs). See, for example, Robson, R. J. Chem. Soc., Dalton Trans. 2000, 3735-3744; Férey, G. J. Solid State Chem., 2000, 152, 37-48; Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O'Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319-330; Chun, H.; Dybtsev, D. N.; Kim, H.; Kim, K. Chem. Eur. J. 2005, 11, 3521-3529; Metal-Organic Frameworks: Design and Application; MacGillivray, L. R., Ed.; Wiley-VCH: Weinheim, Germany, 2010; Kitagawa, S.; Kitaura, R.; Noro, S.-I. Angew. Chem., Int. Ed. 2004, 43, 2334-2375; Férey, G. Chem. Soc. Rev. 2008, 37, 191-214, each of which is incorporated by reference in its entirety. As such, considerable effort has been dedicated to ascertaining the ideal CO₂-MOF interactions/energetics, but minimal systematic studies of finely-tuned MOFs have been reported. See, for example, Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Chem. Rev. 2012, 112, 724-781; Vaidhyanathan, R.; Iremonger, S. S.; Shimizu, G. K. H.; Boyd, P. G.; Alavi, S.; Woo, T. K. Science 2010, 330, 650-653, each of which is incorporated by reference in its entirety.

Development and isolation of novel MBBs can facilitate the rational construction of targeted functional MOFs. See, for example, Liu, Y.; Eubank, J. F.; Cairns, A. J.; Eckert, J.; Kravtsov, V. Ch.; Luebke, R.; Eddaoudi, M. Angew. Chem., Int. Ed. 2007, 46, 3278-3283, each of which is incorporated by reference in its entirety. The discovery of novel modular and rigid inorganic MBBs and establishing reaction conditions that permit to generate a specific inorganic MBB consistently in situ can be a vital criterion/prerequisite for the prospective design and rational construction of desired MOFs.

With the aim to construct porous MOFs with high localized charge density, a potential attribute to promote/enhance the CO₂ sorption energetics, porous MOFs with high localized charge density, a potential attribute to promote/enhance the CO₂ sorption energetics, can be prepared based on metal-ligand directed assembly of electron-rich RE metal ions and non-centrosymmetric hetero-functional ligands containing carboxylate and terazolate moieties. Hexanuclear RE-based (Tb³⁺/Y³⁺) MBBs, generated in situ, to construct a series of 12-connected MOFs can possess face centered cubic (fcu) topology. The MBBs are bridged in a linear fashion through an assortment of fluoro and/or tetrazolate functionalized organic ligands, as outlined in Scheme 1. Systematic gas sorption studies on these materials have elucidated the effects of distinctive parameters on CO₂-MOF sorption energetics.

EXAMPLES

A Series of Fcu-MOFs Based on Rare-Earth Metals and Functional Ligands

Reactions are based on solvothermal reactions between RE metal salts (RE=Y, Tb) and asymmetric hetero-functional ditopic linkers (e.g., 2-fluoro-4-(1H-tetrazol-5-yl)benzoic acid (H₂FTZB) and 4-(1H-tetrazol-5-yl)benzoic acid (H₂TZB)) in various solvent mixtures. Reaction between H₂FTZB and Tb(NO₃)₃.5H₂O in an N,N-dimethylformamide(DMF)/ethanol/chlorobenzene solution yielded transparent polyhedral crystals, formulated by single-crystal x-ray diffraction (SCXRD) studies as [(CH₃)₂NH₂]₂[Tb₆(μ₃-OH)₈(FTZB)₆(H₂O)₆].(H₂O)₂₂ (1).

Compound 1 crystallizes in the cubic space group Fm-3m. In the crystal structure of 1, each Tb³⁺ metal ion is surrounded by four μ₃-OH groups, four oxygen and/or nitrogen atoms from statistically disordered carboxylate groups and/or tetrazolate rings from four independent FTZB²⁻ ligands, leaving the ninth coordination site occupied by a water molecule (FIG. 1). The adjacent Tb ions are bridged via μ₃-OH and deprotonated carboxylate and/or tetrazolate groups in a bis-monodentate fashion to give a [Tb₆(μ₃-OH)₈(O₂C—)₆(N₄C—)₆] MBB. Each hexanuclear MBB is bridged through FTZB²⁻ to produce a 3-periodic MOF.

Structural/topological analysis of the resulting crystal structure reveals that 1 is a MOF with the face-centered cubic (fcu) topology (i.e., an fcu-MOF) constructed from the bridged hexanuclear clusters, [Tb₆(μ₃-OH)₈(O₂C—)₆(N₄C—)₆] MBBs, where the carbon atoms of the coordinated carboxylate and tetrazolate moieties, acting as points of extension, coincide with the cuboctahedron vertex figure of the quasiregular fcu net, the only 12-connected edge transitive net. Edge transitive nets possess only one kind of edge, and are ideal targets in crystal chemistry. See, for example, Friedrichs, O. D.; O'keeffe, M.; Yaghi, O. M. Acta Crystallogr. 2003, A59, 22-27; Friedrichs, O. D.; O'keeffe, M.; Yaghi, O. M. Acta Crystallogr. 2003, A59, 515-525; Robinson, S. A. K.; Mempin, M.-V. L.; Cairns, A. J.; Holman, K. T. J. Am. Chem. Soc. 2011, 133, 1634-1637; Masciocchi, N.; Galli, S.; Colombo, V.; Maspero, A.; Palmisano, G.; Seyyedi, B.; Lamberti, C.; Bordiga, S. J. Am. Chem. Soc. 2010, 132, 7902-7904, each of which is incorporated by reference in its entirety. Replacement of the metal salt with Y(NO₃)₃.6H₂O in the same reaction mixture, resulted in the analogous fcu-MOF, [(CH₃)₂NH₂]₂[Y₆(μ₃-OH)₈(FTZB)₆(H₂O)₆].(H₂O)₅₂ (2). Similar reaction conditions for the non-fluorinated linker, H₂TZB, resulted in clear solutions. However, introduction of a fluorinated modulator, 2-fluorobenzoic acid, has permitted the successful construction of the desired TZB-based isostructural fcu-MOF, [(CH₃)₂NH₂]₂[Tb₆(μ₃-OH)₈(TZB)₆(H₂O)₆].x(solvent) (3), as determined by SCXRD studies. Under the present reaction conditions, a fluoro-substituent located in the alpha (a) position relative to the carboxylate moiety can be necessary for the formation of the 12-connected RE-based MBB. The present hexanuclear clusters, based on mixed carboxylates and tetrazolates, are unprecedented, though a corresponding pure carboxylate molecular cluster based on cerium recently appeared in the open literature. See, for example, Mereacre, V.; Ako, A. M.; Akhtar, M. N.; Lindemann, A.; Anson, C. E.; Powell, A. K. Helv. Chim. Acta 2009, 92, 2507-2524; Das, R.; Sarma, R.; Baruah, J. B. Inorg. Chem. Comm. 2010, 13, 793-795, each of which is incorporated by reference in its entirety.

Occurrence of other analogous hexanuclear clusters in MOF chemistry is limited to a single Zr-based 12-coordinate MBB, where isostructural Zr^(IV)-based fcu-MOFs (e.g., UiO-66) based on [Zr₆(O)₄(OH)₄(O₂C—)₁₂] MBBs are linked together via linear homo-functional dicarboxylate ligands. See, for example, Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. J. Am. Chem. Soc. 2008, 130, 13850-13851; Schaate, A.; Roy, P.; Godt, A.; Lippke, J.; Waltz, F.; Wiebcke, M. and Behrens, P. Chem. Eur. J. 2011, 17, 6643-6651, each of which is incorporated by reference in its entirety.

fcu-MOFs based on RE metals can be constructed, and the [RE₆(μ₃-OH)₈(O₂C—)₆(N₄C—)₆] MBB, RE=Tb and Y can be consistently generated in situ. Such attributes combined with the fact that the fcu net is the only edge transitive net for the assembly of 12-connected cuboctahedron building units, permit the practice of reticular chemistry par excellence, rational MOF design, and thus access to a new MOF platform based on the fcu topology, where the metal ions and ligand functional groups and size to perform a systematic study on the effect of the structural changes on CO₂-MOF energetics can be methodically modified.

The fcu-MOF structure encloses two polyhedral cages, i.e., octahedral and tetrahedral, with effective accessible diameters estimated to be, in the case of compound 1, 14.5 and 9.1 Å (considering van der Waals radii), respectively. Access to the cages is permitted through shared triangular windows, ca. 5-6 Å, which are of suitable size for the adsorption of small gas molecules, e.g., Ar, H₂, CO₂, etc. The corresponding solvent accessible free volumes for 1 and 2 were estimated to be 63.0% and 63.8%, respectively, by summing voxels more than 1.2 Å away from the framework using PLATON software. See, for example, Spek, A. L. Acta Crystallogr. 1990, 46, c34, which is incorporated by reference in its entirety.

In order to achieve maximum and accurate sorption results, the phase purity of the porous material can first be verified. The phase purity of the bulk crystalline materials for 1 and 2 was independently confirmed by similarities between the calculated and as-synthesized powder X-ray diffraction (PXRD) patterns (FIGS. 6A and 6B). In addition, both compounds also show favorable water and thermal stability (FIGS. 2, 11 and 16), which is an important parameter for potential practical deployment of porous MOFs in carbon capture applications.

Argon gas adsorption studies performed on the methanol-exchanged samples show fully reversible type-I isotherms, representative of microporous materials (FIGS. 20 and 23). The apparent BET surface area and pore volume for 1 and 2 were estimated to be 1220 m² g⁻¹ and 0.51 cm³ g⁻¹, and 1310 m² g⁻¹ and 0.56 cm³ g⁻¹, respectively.

In order to evaluate the performance of compounds 1 and 2, an initial H₂ adsorption study at low pressure was performed. The H₂ adsorption uptake was assessed to be 1.96 and 2.19 wt % at 760 torr and 77 K (FIG. 21(a) and S24(a)), while

_(st) for H₂ was determined and estimated to be 8.7 and 9.2 kJ mol⁻¹ at zero coverage for 1 and 2, respectively (FIGS. 21(b) and 24(b)).

To further this study, the CO₂ sorption was investigated, and it was found that 1 and 2 reversibly adsorb a significant amount of CO₂ under ambient conditions, i.e., 3.5 mmol g⁻¹ (15.6%) and 4.1 mmol g⁻¹ (18.1%), respectively, at 298 K and 760 torr (FIG. 3(a)). Interestingly and in contrast to most MOFs, a steep slope is observed in the low pressure region for both materials, a feature that is indicative of enhanced CO₂-MOF interactions. Indeed, the

_(st) for CO₂ calculated from the corresponding variable temperature adsorption isotherms was 58.1 and 46.2 kJ mol⁻¹, for 1 and 2, respectively, at low loading (FIG. 3(b)). In fact, these results are discerned as amongst the highest reported thus far for fully reversible CO₂ sorption on MOFs in the absence of any post-synthetic modification and/or surface area reduction. The accuracy of the

_(st) determination was confirmed across the entire loading range by verifying the linearity of CO₂ adsorption isosters (FIG. 26). At the exception of Mg-MOF-74, the CO₂ uptake at low pressure (0.01 bar and 298 K) for 1 and 2 (Table 1) is the highest reported thus far for MOFs (including amine-functionalized MOFs) with relatively fast CO₂ adsorption kinetics (FIG. 54).

TABLE 1 CO₂ uptake in compounds 1 and 2 as compared to other MOFs reported in the literature. CO₂ uptake at 0.01 bar Q_(st) at low coverage MOFs (mmol g⁻¹) (kJ mol⁻¹) Compound 1 0.33 58.1 Compound 2 0.62 46.2 Mg-MOF-74^([a]) 1.5 47 Mmen-Cu BTTri^([b]) 0.023 96 ^([a])Mg-MOF-74: Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2008, 130, 10870-10871. ^([b])Mmen-Cu BTTri: McDonald, T. M.; D'Alessandro, D. M.; Krishna, R.; Long, J. R. Chem. Sci. 2011, 2, 2022-2028.

In order to pinpoint and understand the different energetic levels associated with the unique CO₂ adsorption properties observed in 1 and 2, particularly at low pressures, we performed an in-depth

_(st) analysis study using a multiple site Langmuir model (MSL). In fact, three energetic sites were clearly identified and derived from the best fit and convergence obtained when using the triple site Langmuir model (FIG. 27). The observed energies for sites I and III were found to be identical in 1 and 2, ca. 60 and 25-26 kJ mol⁻¹, respectively. The former energetic site can be attributed to the localized high concentration of charge density resultant from the mutual presence of both a fluoro substituent and the nitrogen-rich tetrazolate moiety in proximal vicinity of the open metal site, while site III is simply due to the effect of pore filling. See, for example, Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Chem. Rev. 2012, 112, 724-781; Lin, J.-B.; Zhang, J.-P.; Chen, X.-M. J. Am. Chem. Soc. 2010, 132, 6654-6656; Lin, Q.; Wu, T.; Zheng, S.-T.; Bu, X.; Feng, P. J. Am. Chem. Soc. 2012, 134, 784-787; Burd, S. D.; Ma, S. Q.; Perman, J. A.; Sikora, B. J.; Snurr, R. Q.; Thallapally, P. K.; Tian, J.; Wojtas, L.; Zaworotko, M. J. J. Am. Chem. Soc. 2012, 134, 3663-3666; Luebke, R.; Eubank, J. F.; Cairns, A. J.; Belmabkhout, Y.; Wojtas, L.; Eddaoudi, M. Chem. Commun., 2012, 48, 1455-1457, each of which is incorporated by reference in its entirety. Differences arising from the choice of metal ion are evident in site II, where energetic levels of 47 and 35 kJ mol⁻¹ were determined for compounds 1 and 2, respectively. The recorded

_(st) is likely the average energy of these sites, while the total CO₂ uptake is the summation of adsorption isotherms for sites I, II and III (FIGS. 4B-4D and 28A-28C). The presence of conical pockets (i.e., tripodal and quadrapodal narrow size cavities), decorated with fluoro moieties and tetrazolate groups, can create a high localized charge density and promote synergetic effects favorable for enhanced CO₂ sorption at low loadings. Using site I parameters for compound 1, ideal adsorbed solution theory (IAST; see Myers, A. L. & Prausnitz, J. M. AlChE J. 11, 1965, 121-127, which is incorporated by reference in its entirety) prediction of adsorption at various trace concentration of CO₂ (from 100 ppm to 1%) in a mixture with N₂, mimicking vacuum swing operational mode at various working pressures, revealed an exceptionally high adsorption selectivity (ca. 370) for CO₂ over N₂ (FIG. 5A). This finding was further confirmed experimentally using a column breakthrough test with a CO₂/N₂:0.01/99.99% mixture (FIG. 57), showing an even higher selectivity (ca. 1051).

The H₂ and CO₂, as well as other gas, sorption properties were further investigated at high pressure. It was found that at 77 K and 40 bar 1 and 2 store 3.9 and 4.4 wt % of H₂, respectively, while for CO₂ 7.1 mmol g⁻¹ (31.2%) and 9.3 mmol g⁻¹ (41.1%) were adsorbed, respectively, at 298 K and 25 bar (FIGS. 51A-51F and 52A-52F). Though these values are lower than those recorded for Mg-MOF-74, they are among the highest CO₂ uptakes per surface unit reported at 25 bar. Markedly, when sites I are fully saturated at lower CO₂ pressures, the less energetic sites (II and III) dominate the CO₂ adsorption at moderately higher CO₂ concentration and pressure as reflected by the relatively reduced CO₂/N₂ selectivity to ca. 16 at 10% vs. 370 at 0.01%, as determined by IAST (FIG. 58). The predominance of site I, the CO₂ sorption high energetic site, can permit efficient CO₂ separation at intermediate (10%, flue gas) and high (30-50%, biogas) CO₂ concentration.

The successful isolation of reaction conditions that consistently permit the in situ generation of the [RE₆(μ₃-OH)₈(O₂C—)₆(N₄C—)₆] MBB, and corresponding fcu-MOF platform, offer potential to assess the distinctive role of the fluoro substituent and terazolate moiety on the adsorbate-MOF interactions. Accordingly, various analogous/isoreticular fcu-MOFs were targeted and synthesized, including other RE metal ions (e.g., La³⁺, Eu³⁺ and Yb³⁺) (FIG. 10) and diverse mono-/poly-fluorinated, hetero-/homo-functional, and extended ligands.

In the first example, the organic linker was expanded from H₂FTZB to 3-fluoro-4′-(2H-tetrazol-5-yl)biphenyl-4-carboxylic acid (H₂FTZBP) (Scheme 1) and reacted with Tb or Y nitrate salts to give the expected isoreticular compounds, [(CH₃)₂NH₂]₂[Tb6(μ₃-OH)₈(FTZBP)₆(H₂O)₆].x(solvent) (4) or [(CH₃)₂NH₂]₂[Y₆(μ₃-OH)₈(FTZBP)₆(H₂O)₆].x(solvent) (5), respectively. As expected, the analogous fluorinated dicarboxylate linker, 3-fluorobiphenyl-4,4′-dicarboxylate (FBPDC, Scheme 1), which is generated in situ via hydrolysis of 4′-cyano-3-fluorobiphenyl-4-carboxylic acid, and 3,3′-difluorobiphenyl-4,4′-dicarboxylic acid (H₂DFBPDC, Scheme 1) react with Tb to give the isoreticular analog of 1, denoted as [(CH₃)₂NH₂]₂[Tb6(μ₃-OH)₈(FBPDC)₆(H₂O)₆].x(solvent) (6) and [(CH₃)₂NH₂]₂[Tb₆(μ₃-OH)₈(DFBPDC)₆(H₂O)₆].x(solvent) (7), respectively. The CO₂ sorption properties were assessed for compounds 3-7, and, as expected, fcu-MOFs constructed from the elongated fluorinated hetero-functional ligand (i.e., 4 and 5) revealed a lower adsorption capacity and reduced

_(st) values (36.7 and 27.2 kJ mol⁻¹, respectively) compared to the parent fcu-MOF based on the shorter and conjugated FTZB ligand. This study clearly supports that reducing the electronic density (by increasing the distance between the fluoro and tetrazolate substituents; i.e., by not having both of them on the same phenyl ring) affords a weaker CO₂-framework affinity, which is also directly reflected by the reduced CO₂ uptake. Likewise, 3, 6, and 7, from TZB²⁻, FBPDC²⁻ and DFBPDC²⁻ ligands, respectively, have less localized electronic charge density when compared to 1 based on the more polarized FTZB²⁻ ligand, and thus show reduced CO₂ adsorption uptakes and relatively lower

_(st) values for CO₂ adsorption at low loading (39.1-46.6 vs 58.1 kJ mol⁻¹ for 1). Additionally, MSL analysis performed on the CO₂ sorption data for 3 and 6 showed that the best fit and convergence was attained only when the dual site Langmuir was applied (FIG. 48), suggesting the presence of merely two energetic adsorption sites instead of the three energetic sites originally observed in the parent tetrazolate-based fcu-MOFs (e.g., 1 and 2).

Given the unique structural features of this RE-based fcu-MOF platform, the following synergistic combination of effects is likely responsible for the notable CO₂ capacity and high affinity towards CO₂. These include (i) a high concentration of localized electron-rich vacant metal sites; (ii) the presence of polar groups (i.e., —F, —OH) and nitrogen-rich tetrazolate rings in a confined narrow space and at a proximal vicinity of the open metal sites, favoring multiwall (multi-sites) interactions with a single CO₂ molecule, allowing their interaction with CO₂ in a synergistic fashion.

Reaction conditions that consistently permit the in situ generation of the RE₆(μ₃-OH)₈(O₂C—)₆(N₄C—)₆] and [RE₆(μ₃-OH)₈(O₂C—)₁₂] hexanuclear MBBs were isolated and successfully employed for the construction of a series of robust and water stable 12-connected RE-based fcu-MOFs based on fluorinated/non-fluorinated and hetero-/homo-functional ligands. Trivalent RE metal clusters can be assembled into highly-connected MOFs, in this case fcu-MOFs, displaying diverse adsorption energetics toward CO₂. The utilization of polarized ligands containing tetrazolate and fluoro moieties afforded enhanced sorption energetic and uptakes due to their unique special positioning, in a narrow proximal vicinity of the open metal sites, offered by the unique fcu-MOF structure. The high CO₂ affinity vs. N₂, particularly at low pressure, as well as the favorable tolerance to water and high thermal stability, certainly renders 1 and 2 promising prospective adsorbents for low CO₂ concentration purification involving multicomponent gas adsorption. Studies are underway to further employ the newly isolated 12-connected [[RE₆(μ₃-OH)₈(O₂C—)₆(N₄C—)₆] and [RE₆(μ₃-OH)₈(O₂C—)₁₂] MBBs for the construction of highly connected MOFs based on hetero-/homo-trifunctional and tetrafunctional ligands with the main objective to increase the concentration per unit surface of the highly energetic sites for CO₂ sorption in a wide range of pressures.

EXAMPLES

Materials and Methods. The organic ligands used in this study, i.e., 2-fluoro-4-(1H-tetrazol-5-yl) benzoic acid (H₂FTZB) and 4-(2Htetrazol-5-yl) benzoic acid (H₂TZB), were synthesized from 4-cyano-2-fluorobenzoic acid and 4-cyanobenzoic acid, respectively, with 67 and 74% yields using the Demko-Sharpless method.¹⁴ The organic ligand 3-fluoro-4′-(2H-tetrazol-5-yl)biphenyl-4-carboxylic acid (H₂FTZBP) was synthesized from 4′-cyano-3-fluorobiphenyl-4-carboxylic acid according to literature methods.¹⁵ The organic ligand 3,3′-difluorobiphenyl-4,4′-dicarboxylic acid (H₂DFBPDC) was synthesized from the following Suzuki homocoupling reaction: A mixture of 4-borono-2-fluorobenzoic acid (2.0 g, 10 mmol), potassium carbonate (1.5 g) and 5% unreduced palladium on carbon (2.0 g) in ethanol (20 mL) was heated at 85° C. for 24 h under nitrogen. The mixture was filtered through a Celite pad, and the solvent was evaporated. Five milliliters of 1.0 M sodium hydroxide were added to dissolve the solid. The solution was acidified by 1.0 M HCl after filtering and extracted in ethyl acetate, dried over Na₂SO₄, and filtered, and the volatiles were removed under reduced pressure to yield H₂DFBPDC as a white crystalline solid (0.5 g, 36% yield). ¹H NMR (500 MHz, DMF-d7): δ=7.97 (t, J=7.6 Hz, 2H), 7.69 (q, J=6.4 Hz, 2H), 7.31-7.39 (m, 2H). All other reagents were obtained from commercial sources and used without further purification.

Fourier-transform infrared (FT-IR) spectra (4000-600 cm⁻¹) were collected in the solid state on a Nicolet 700 FT-IR spectrometer. The peak intensities are described in each of the spectra as very strong (vs), strong (s), medium (m), weak (w), broad (br) and shoulder (sh).

Powder X-ray diffraction (PXRD) measurements were performed on a PANalytical X′ Pert PRO MPD X-ray diffractometer at 45 kV, 40 mA for Cu Kα (λ=1.5418 Å) equipped with a variable-temperature stage, with a scan speed of 2°/min. The sample was held at the designated temperatures for at least 10 min between each scan. High resolution dynamic thermogravimetric analysis (TGA) were performed under a continuous N₂ flow and recorded on a TA Instruments hi-res TGA Q500 thermogravimetric analyzer with a heating rate of 5° C. per minute. Low pressure gas sorption measurements were performed on a fully automated Autosorb-1C gas sorption analyzer (Quantachrome Instruments). High pressure gas sorption studies were performed on a magnetic suspension balance marketed by Rubotherm (Germany). The SEM image was recorded on a Quanta 600 FEG scanning electron microscope at 30 kV, and the optical images were taken on a CMM-55 microscope. Water vapor sorption measurements were conducted at room temperature on a VTI-SA symmetrical vapor sorption analyzer. Synthesis of Compounds. Synthesis of Tb-FTZB-MOF (1).

H₂FTZB (13.6 mg, 0.0653 mmol), Tb(NO₃)₃.5H₂O (18.9 mg, 0.0435 mmol), DMF (1.0 mL), C₂H₅OH (0.5 mL), and chlorobenzene (0.5 mL) were combined in a 20 mL scintillation vial, sealed and heated to 115° C. for 72 h and cooled to room temperature. The colorless polyhedral crystals were collected and air dried. FT-IR (4000-600 cm⁻¹): 3379 (br), 1651 (s), 1611 (m), 1388 (vs), 1251 (w), 1097 (m), 905 (m), 797 (m), 746 (m), 656 (m).

Synthesis of Y-FTZB-MOF (2). H₂FTZB (13.6 mg, 0.0653 mmol), Y(NO₃)₃.6H₂O (16.7 mg, 0.0435 mmol), DMF (1.0 mL), C₂H₅OH (0.5 mL), and chlorobenzene (0.5 mL) were combined in a 20 mL scintillation vial, sealed and were heated to 115° C. for 72 h. The colorless polyhedral crystals were collected and air-dried. FT-IR (4000-600 cm⁻¹): 3385 (br), 1658 (s), 1612 (m), 1391 (vs), 1204 (w), 1090 (s), 904 (s), 800 (m), 750 (m), 656 (m).

Synthesis of Tb-TZB-MOF (3). H₂TZB (16.5 mg, 0.087 mmol), Tb(NO₃)₃.5H₂O (18.9 mg, 0.0435 mmol), 2-fluorobenzoic acid (48.7 mg, 0.348 mmol), DMF (1.0 mL), C₂H₅OH (1.5 mL) were combined in a 10 mL microwave tube, sealed and heated to 115° C. for 72 h and cooled to room temperature. The colorless polyhedral crystals were collected and air-dried. FT-IR (4000-600 cm⁻¹): 3358 (br), 1656 (s), 1603 (vs), 1659 (s), 1497 (w), 1397 (vs), 1281 (w), 1255 (w), 1176 (w), 1099 (s), 1058 (w), 1011 (m), 878 (w), 840 (w), 801 (m), 751 (s), 701 (w), 663 (w).

Synthesis of Tb-FTZBP-MOF (4). H₂FTZBP (24.7 mg, 0.087 mmol), Tb(NO₃)₃.5H₂O (18.9 mg, 0.0435 mmol), DMF (1.0 mL), C₂H₅OH (0.5 mL), and chlorobenzene (0.5 mL) were combined in a 20 mL scintillation vial, sealed and heated to 115° C. for 72 h and cooled to room temperature. The brown polyhedral crystals were collected and air-dried. FT-IR (4000-600 cm⁻¹): 3358 (br), 1650 (vs), 1610 (m), 1411 (m), 1385 (m), 1254 (w), 1099 (s), 1009 (w), 905 (w), 843 (w), 796 (m), 765 (m), 660 (w).

Synthesis of Y-FTZBP-MOF (5). H₂FTZBP (24.7 mg, 0.087 mmol), Y(NO₃)₃.6H₂O (16.8 mg, 0.0435 mmol), DMF (1.0 mL), C₂H₅OH (0.5 mL), and chlorobenzene (0.5 mL) were combined in a 20 mL scintillation vial, sealed and heated to 115° C. for 72 h and cooled to room temperature. The brown polyhedral crystals were collected and air-dried. FT-IR (4000-600 cm-1): 3363 (br), 1657 (vs), 1611 (v), 1499 (m), 1412 (m), 1385 (s), 1251 (w), 1097 (s), 1058 (w), 1007 (m), 906 (w), 845 (w), 796 (m), 765 (m), 660 (w).

Synthesis of Tb-FBPDC-MOF (6). 4-Cyano-3-fluorobiphenyl-4-carboxylic acid (41.9 mg, 0.174 mmol), Tb(NO₃)₃.5H₂O (37.8 mg, 0.087 mmol), DMF (1.5 mL), C₂H₅OH (0.5 mL), and chlorobenzene (0.5 mL) were combined in a 20 mL scintillation vial, sealed and heated to 115° C. for 72 h and cooled to room temperature. The colorless polyhedral crystals were collected and air-dried. FT-IR (4000-600 cm⁻¹): 3350 (br), 1655 (w), 1584 (vs), 1528 (w), 1382 (vs), 1188 (w), 1109 (m), 1014 (w), 907 (m), 846 (m), 779 (s), 697 (w), 662 (w).

Synthesis of Tb-DFBPDC-MOF (7). H₂DFBPDC (18.2 mg, 0.065 mmol), Tb(NO₃)₃.5H₂O (18.9 mg, 0.0435 mmol), DMF (1.0 mL), C₂H₅OH (0.5 mL), and chlorobenzene (0.5 mL) were combined in a 20 mL scintillation vial, sealed and heated to 115° C. for 60 h and cooled to room temperature. The colorless polyhedral crystals were collected and air-dried. FT-IR (4000-600 cm⁻¹): 3338 (br), 1651 (w), 1582 (vs), 1493 (w), 1528 (w), 1385 (vs), 1253 (w), 1209 (w), 1102 (m), 1061 (w), 954 (w), 861 (m), 843 (m), 784 (m), 695 (m).

Low-Pressure Gas Adsorption Measurements

Low pressure gas adsorption studies were conducted on a fully automated micropore gas analyzer Autosorb-1C (Quantachrome Instruments) at relative pressures up to 1 atm. The cryogenic temperature was controlled using liquid nitrogen and argon baths at 77 K and 87 K, respectively. The bath temperature for the CO₂ sorption measurements was controlled using a recirculating bath containing an ethylene glycol/H₂O mixture. The apparent surface areas were determined from the argon adsorption isotherms collected at 87 K by applying the Brunauer-Emmett-Teller (BET) and Langmuir models. Pore size analyses were performed using a cylindrical/spherical NLDFT pore model system by assuming an oxidic (zeolitic) surface. The determination of the isosteric heats of adsorption (

_(st)) for H₂ and CO₂ was estimated by applying the Clausius-Clapeyron expression using the H₂ sorption isotherms measured at 77 K and 87 K and the CO₂ isotherms measured at 258, 273 and 298 K unless otherwise noted.

Homogenous microcrystalline samples of compounds 1-7 were activated by washing the as-synthesized crystals with 3×20 mL of DMF followed by solvent exchange in methanol (Compounds 1-3) or ethanol (compounds 4-7) for 3 days. The solution was refreshed several times daily during this time period. In a typical experiment, 30 to 40 mg of each activated sample was transferred (dry) to a 6-mm large bulb glass sample cell and firstly evacuated at room temperature using a turbo molecular vacuum pump and then gradually heated to 160° C. for 1, 2, 3 and 7, 120° C. for 4-5 (increasing at a rate of 1° C./min), held for 16 h and cooled to room temperature. Data are presented in Table 2.

TABLE 2 Low pressure sorption data summary for compounds 1-7. H₂ CO₂

 for BET Langmuir P.V. Uptake^(a)

 for H₂ Uptake^(b) CO₂ Compound (m² g⁻¹⁾ (m² g⁻¹) (cm³ g⁻¹) (wt %) (kJ mol⁻¹) (mmol g⁻¹) (kJ mol⁻¹) 1 1220 1510 0.51 1.96 8.69-5.39 7.53, 5.86 58.1-25.0 and 3.54 2 1310 1640 0.56 2.19 9.18-5.68 8.33, 6.46 46.1-24.0 and 4.12 3 904 1145 0.39 1.47 8.45-5.57 5.96, 4.53 46.6-23.8 and 2.50 4 2200 2560 0.86 1.34 8.65-5.06 3.93, 2.75 36.7-20.3 and 1.64 5 2410 2820 0.94 1.52 8.52-4.91 4.01, 2.81 27.2-19.5 and 1.59 6 1940 2330 0.78 1.37 6.84-5.06 3.50, 2.40 39.1-18.5 and 1.37 7 1854 2161 0.72 1.36 7.05-4.99 3.91, 2.63 41.6-20.7 and 1.36 ^(a)uptake at 77 K and 760 torr; ^(b)= CO₂ uptake measured at 258, 273 and 298 K respectively. CO₂ Adsorption 

 Analysis on Compounds 1 and 2 Using Multiple-site Langmuir model (MSL) n=n _(sat1) *b ₁ p/1+b ₁ p+n _(sat2) *b ₂ p/1+b ₂ p+ . . . +n _(sati) *b _(i) p/1+b _(i) p  Equation for MSL:

The best fit and convergence were obtained with the triple site Langmuir (TSL) model. The parameters extracted from the best TSL fit were used to recalculate the adsorption isotherms and the evolution of the

_(st) for each energetic site (site I, site II and site III) using the Clausius-Clapeyron equation.

TABLE S2 Triple site Langmuir parameters (Compound 1) Temperature/K Parameters 258 273 298 Adsorption n_(sat1) 0.33956 0.27756 0.26915 Site I b₁ 147.49445 48.15585 5.50219 Adsorption n_(sat2) 0.37026 0.40104 0.42827 Site II b₂ 4.91024 1.58224 0.22432 Adsorption n_(sat3) 10.86539 11.18523 15.24316 Site III b₃ 0.01703 0.00865 0.00231

TABLE S3 Triple site Langmuir parameters (Compound 2) Temperature/K Parameters 258 273 298 Adsorption n_(sat1) 0.39195 0.35252 0.29521 Site I b₁ 259.98806 80.03201 23.96898 Adsorption n_(sat2) 0.52812 0.5334 0.62601 Site II b₂ 9.75852 2.64722 0.83725 Adsorption n_(sat3) 11.82402 12.0789 12.99557 Site III b₃ 0.01696 0.00859 0.00326

High-Pressure Gas Adsorption Measurements

Adsorption equilibrium measurements for the pure gases were performed using a Rubotherm gravimetric-densimetric apparatus (Bochum, Germany) (FIG. 50), composed mainly of a magnetic suspension balance (MSB) and a network of valves, mass flowmeters and temperature and pressure sensors. The MSB overcomes the disadvantages of other commercially available gravimetric instruments by separating the sensitive microbalance from the sample and the measuring atmosphere and is able to perform adsorption measurements across a wide pressure range, i.e. 0 to 200 bar. Moreover, the adsorption temperature can be controlled in the range of 77 K to 423 K. In a typical adsorption experiment, the adsorbent is precisely weighed and placed in a basket suspended by a permanent magnet through an electromagnet. The cell in which the basket is housed is then closed and vacuum or high pressure is applied. The gravimetric method allows the direct measurement of the reduced gas adsorbed amount Ω. Correction for the buoyancy effect is required to determine the excess adsorbed amount using equation 1, where V_(adsorbent) and V_(ss) refer to the volume of the adsorbent and the volume of the suspension system, respectively. These volumes are determined using the helium isotherm method by assuming that helium penetrates in all open pores of the materials without being adsorbed. The density of the gas is determined using Refprop equation of state (EOS) database and checked experimentally using a volume-calibrated titanium cylinder. By weighing this calibrated volume in the gas atmosphere, the local density of the gas is also determined. Simultaneous measurement of adsorption capacity and gas phase density as a function of pressure and temperature is therefore possible. The excess uptake is the only experimentally accessible quantity and there is no reliable experimental method to determine the absolute uptake. For this reason, only the excess amounts are considered in this work. Ω=m _(excess)−ρ_(gas)(V _(absorbant) +V _(ss))  (1)

The pressure is measured using two Drucks high pressure transmitters ranging from 0.5 to 34 bar and 1 to 200 bar, respectively, and one low pressure transmitter ranging from 0 to 1 bar. Prior to each adsorption experiment, about 100 mg to 300 mg sample is outgassed at 433 K at a residual pressure 10⁻⁴ mbar. The temperature during adsorption measurements is held constant by using a thermostated circulating fluid.

FIG. 55 shows the selectivity of CO₂ over N₂ at 298 K, calculated (using IAST)[c] from levels of a few ppm to 1%, assuming CO₂ interaction with compound 1 are completely governed by adsorption on site I. The selectivity was calculated assuming different total pressures for the mixtures (i.e., 0.5, 1 and 2 bar). The purpose of the total pressure variation is to mimic vacuum swing adsorption (VSA) regeneration mode conditions supposing 0.2, 0.5 and 1 bar as the working adsorption pressure and vacuum as the desorption pressure. As was expected, the CO₂ selectivity over N₂ was high (ca. 370) in the domain when interaction with site I are the most dominant. Prediction of CO₂/N₂ selectivity at variable total pressures from 0.5 bar and up to 2 bar showed that the CO₂/N₂ separation decreased by increasing the total pressure and concentration due to the quick saturation of most of energetics sites (site I) available. Therefore a way to maintain high selectivity is to increase the density of site I. In order to confirm this finding, breakthrough adsorption experiments were carried out using a CO₂/N₂ mixture containing 1000 ppm of CO₂ at 298 K and a total pressure of 1 bar. The purpose of using such low concentration is to explore experimentally the separation performance of the compound 1 where the adsorption is mostly governed by the most energetic site (site I).

Interestingly, the breakthrough test shows that the CO₂ was retained in the bed for ca. 5250 s while N₂ breakthrough occurred almost after few second (FIG. 56). The gas uptake for CO₂ and N₂ at breakthrough was 0.262 and 0.249 mmol/g. Therefore, the CO₂/N₂ selectivity was exceptionally high (ca. 1051) exceeding the predicted selectivity using IAST. This finding is extremely important as it shows that materials with high density of adsorption site I will certainly lead to suitable separation agents for CO₂ removal from gas streams with even higher CO₂ concentration (10-30%) in order to produce useful commodities such as CH₄, O₂ and H₂ with higher efficiency. Ongoing work is focusing on the design of new MOFs with such attributes.

Single Crystal X-ray Crystallography. Single-crystal X-ray diffraction data were collected using a Bruker-AXS SMART-APEX2 CCD diffractometer (Cu Kα, λ=1.54178 Å) for compounds 1 and 2, Bruker X8 PROSPECTOR APEX2 CCD (Cu Kα, λ=1.54178 Å) for compounds 3 and 5-7, and Oxford Supernova Atlas CCD (Mo Kα=0.71073 Å) for compound 4. Indexing was performed using APEX2 (Difference Vectors method). 16 Data integration and reduction were performed using SaintPlus 6.01. Bruker SAINT, Data Reduction Software; Bruker AXS, Inc.: Madison, Wis., 2009, which is incorporated by reference in its entirety. Absorption correction was performed by multiscan method implemented in SADABS. Sheldrick, G. M. SADABS, Program for Empirical Absorption Correction; University of Gottingen: Gottingen, Germany, 2008, which is incorporated by reference in its entirety. Space groups were determined using XPREP implemented in APEX2. Bruker APEX2; Bruker AXS, Inc.: Madison, Wis., 2010, which is incorporated by reference in its entirety. Structures were solved using SHELXS-97 (direct methods) and refined using SHELXL-97 (full-matrix least-squares on F2) contained in APEX216 and WinGX v1.70.01 programs packages. See, for example, (a) Farrugia, L. J. Appl. Crystallogr. 1999, 32, 837-838, and Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal; University of Gottingen: Gottingen, Germany, 1997. (c) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473. (d) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112-122, each of which is incorporated by reference in its entirety. CrysAlis Pro package was used to process diffraction images for compound 4. CrysAlis Pro; Oxford Diffraction: Abingdon, U. K., 2009, which is incorporated by reference in its entirety. For all compounds the ligand moiety was disordered and atoms were refined using geometry restraints. Restraints were also used to refine anisotropic displacement parameters of disordered atoms. Disordered cations and solvent molecules were refined isotropically. Relatively high residual electron density observed in a μ-OH position (leading to very small value of thermal parameters for μ-OH oxygen) are most likely attributed to “electron transfer ( . . . ) directed from d-orbitals to the oxygen 2p orbitals”, which is observed in yttrium-oxide clusters. Pramann, A.; Nakamura, Y.; Nakijama, A.; Kaya, K. J. Phys. Chem. A 2001, 105, 7534-7540, which is incorporated by reference in its entirety. Hydrogen atoms were placed in geometrically calculated positions and included in the refinement process using riding model with isotropic thermal parameters: U_(iso)(H)=1.2Ueq(-OH, —CH).

The crystal of compound 7 was twinned, twinning law [−0.66/−0.33/0.66][0.66/−0.66/0.33] [0.33/0.66/0.66]. Two reciprocal lattices have been identified using XPREP (APEX2); diffraction data have been integrated using SAINT and scaled/corrected using TWINABS. Sheldrick, G. M. TWINABS; Bruker AXS, Inc.; Madison, Wis., 2002, which is incorporated by reference in its entirety. Refinement has been carried using HKLF 5 style reflection data containing reflection from both domains (BASF=0.12). Distance restraints have been used to refine disordered benzene rings.

Disordered atoms have been refined isotropically. For compounds 3-7, the contribution of heavily disordered solvent molecules was treated as diffuse using Squeeze procedure implemented in Platon program. Spek, T. L. Acta Crystallogr. 1990, A46, 194-201, which is incorporated by reference in its entirety. Crystal data and refinement conditions are shown in Tables 3-11.

TABLE 3 Selected Low Pressure Sorption Data for Compounds 1-7 compound 1 2 3 4 5 6 7 BET (m² g⁻¹) 1220 1310 904 2200 2410 1940 1854 PV (cm³ g⁻¹) 0.51 0.56 0.39 0.86 0.94 0.78 0.72 CO₂ uptake^(a) 7.53, 5.86, 8.33, 6.46, 5.96, 4.53, 3.93, 2.75, 4.01, 2.81 3.50, 2.40, 3.91, 2.63, (mmol g⁻¹) and 3.54 and 4.12 and 2.50 and 1.64 and 1.59 and 1.37 and 1.36

 for CO₂ 58.1-25.0 46.1-24.0 46.6-23.8 36.7-20.3 27.2-19.5 39.1-18.5 41.6-20.7 (kJ mol⁻¹) ^(a)CO₂ uptake at 760 Torr measured at 258, 273, and 298 K, respectively.

TABLE 4 Selected Crystallographic Data and Structural Refinement for Compounds 1-7 compound 1 2 3 4 formula C₅₂H₁₈Tb₆N₂₆O₄₈F₆ C₅₂H₁₈Y₆N₂₆O₇₈F₆ C₄₈H₃₂Tb₆N₂₄O₅₈ C₈₄H₄₂Tb₆N₂₄O₂₆F₆ FW (g mol⁻¹) 2842.44 2902.38 2826.43 2870.94 crystal system cubic cubic cubic cubic space group Fm3m Fm3m Fm3m Fm3m a (Å) 23.5553(2) 23.4365(4) 23.5195(5) 29.5957(3) V (Å³) 13069.7(2) 12873.0(4) 13010.2(5) 25923.0(5) Z, D_(cal) (g cm⁻³) 4, 1.445 4, 1.498 4, 1.439 4, 0.736 θ_(max) (°) 65.74 63.48 67.93 28.27 R_(int) 0.0610 0.0313 0.0380 0.0289 R₁ (I > 2σ(I₀)) 0.0359 0.0395 0.0340 0.0236 wR₂ (all data) 0.1315 0.1183 0.1031 0.0724 GOF 1.099 1.052 1.086 1.072 Δρ_(max)/Δρ_(min) 1.559/−0.396 1.849/−0.597 1.161/−0.493 1.128/−0.532 (e.Å⁻³) compound 5 6 7 formula C₈₄H₄₂Y₆N₂₄O₂₆F₆ C₈₄H₄₂Tb₆O₃₈F₆ C₈₄H₃₆Tb₆O₄₆F₁₂ FW (g mol⁻¹) 2450.88 2726.70 2962.65 crystal system cubic cubic cubic space group Fm3m Fm3m Fm3m a (Å) 29.447(3) 27.5127(12) 27.4756(7) V (Å³)  25535(4) 20825.7(16) 20741.6(9) Z, D_(cal) (g cm⁻³) 4, 0.638 4, 0.870 4, 0.949 θ_(max) (°) 65.64 63.44 67.93 R_(int) 0.0198 0.0442 0.000 R₁ (I > 2σ(I₀)) 0.0337 0.0402 0.0732 wR₂ (all data) 0.1016 0.1125 0.2085 GOF 1.059 1.051 1.051 Δρ_(max)/Δρ_(min) 0.675/−0.283 0.839/−0.716 1.137/−1.339 (e.Å⁻³)

TABLE 5 Crystal data and structure refinement for compound Tb-FTZB-MOF (1) Identification code 1 Empirical formula C₅₂ H₁₈ F₆ N₂₆ O₄₈ Tb₆ Formula weight 2842.44 Temperature 100(2) K Wavelength 1.54178 Å Crystal system, space group Cubic, Fm-3m Unit cell dimensions a = 23.5553(2) Å alpha = 90 deg. b = 23.5553(2) Å beta = 90 deg. c = 23.5553(2) Å gamma = 90 deg. Volume 13069.71(19) Å Z, Calculated density 4, 1.445 Mg/m³ Absorption coefficient 16.373 mm⁻¹ F(000) 5360 Crystal size 0.10 × 0.10 × 0.10 mm Theta range for data collection 5.31 to 65.74 deg. Limiting indices −19 <= h <= 26, −27 <= k <= 27, −27 <= l <= 24 Reflections collected/unique 14184/637 [R(int) = 0.0610] Completeness to theta = 65.74 99.4% Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.2913 and 0.2913 Refinement method Full-matrix least-squares on F² Data/restraints/parameters 626/57/81 Goodness-of-fit on F² 1.099 Final R indices [I > R₁ = 0.0359, wR₂ = 0.1294 2sigma(I)] R indices (all data) R₁ = 0.0401, wR₂ = 0.1315 Largest diff. peak and hole 1.559 and −0.396 e. Å⁻³

TABLE 6 Crystal date and structure refinement for compound Y-FTZB-MOF (2) Identification code 2 Empirical formula C₅₂ H₁₈ F₆ N₂₆ O₇₈ Y₆ Formula weight 2902.38 Temperature 100(2) K Wavelength 1.54178 Å Crystal system, space group Cubic, Fm-3m Unit cell dimensions a = 23.4365(4) Å alpha = 90 deg. b = 23.4365(4) Å beta = 90 deg. c = 23.4365(4) Å gamma = 90 deg. Volume 12873.0(4) Å³ Z, Calculated density 4, 1.498 Mg/m³ Absorption coefficient 4.527 mm⁻¹ F(000) 5696 Crystal size 0.10 × 0.10 × 0.10 mm Theta range for data collection 5.34 to 63.48 deg. Limiting indices −27 <= h <= 25, −18 <= k <= 27, −17 <= l <= 10 Reflections collected/unique 5335/575 [R(int) = 0.0313] Completeness to theta = 63.48 97.0% Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.6603 and 0.6603 Refinement method Full-matrix least-squares on F² Data/restraints/parameters 575/69/88 Goodness-of-fit on F² 1.052 Final R indices [I > R₁ = 0.0395, wR₂ = 0.1160 2sigma(I)] R indices (all data) R₁ = 0.0414, wR₂ = 0.1183 Largest diff. peak and hole 1.849 and −0.597 e. Å⁻³

TABLE 7 Crystal data and structure refinement for compound Tb-TZB-MOF (3) Identification code 3 Empirical formula C₄₈ H₃₂ N₂₄ O₅₈ Tb₆ Formula weight 2826.43 Temperature 100(2) K Wavelength 1.54178 Å Crystal system, space group Cubic. Fm-3m Unit cell dimensions a = 23.5195(5) Å alpha = 90 deg. b = 23.5195(5) Å beta = 90 deg. c = 23.5195(5) Å gamma = 90 deg. Volume 13010.2(5) Å³ Z, Calculated density 4, 1.439 Mg/m³ Absorption coefficient 16.428 mm⁻¹ F(000) 5336 Crystal size 0.10 × 0.10 × 0.10 mm Theta range for data collection 5.32 to 67.93 deg. Limiting indices −26 <= h <= 22, −20 <= k <= 18, −22 <= l <= 27 Reflections collected/unique 10330/624 [R(int) = 0.0380] Completeness to theta = 65.93 99.0% Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.2904 and 0.2904 Refinement method Full-matrix least-squares on F² Data/restraints/parameters 624/1/64 Goodness-of-fit on F² 1.086 Final R indices (I > R₁ = 0.0340, wR₂ = 0.1028 2sigma(I)] R indices (all data) R₁ = 0.0343, wR₂ = 0.1031 Largest diff. peak and hole 1.161 and −0.493 e. Å⁻³

TABLE 8 Crystal data and structure refinement for compound Tb-FTZBP-MOF (4) Identification code 34 Empirical formula C₈₄ H₄₂ F₆ N₂₄O₂₆Tb₆ Formula weight 2870.94 Temperature 200(2) K Wavelength 0.71073 Å Crystal system, space group Cubic, Fm-3m Unit cell dimensions a = 29.5957(3) Å alpha = 90 deg. b = 29.5957(3) Å beta = 90 deg. c = 29.5957(3) Å gamma = 90 deg. Volume 25923.0(5) Å³ Z, Calculated density 4, 0.736 Mg/m³ Absorption coefficient 1.651 mm⁻¹ F(000) 5464 Crystal size 0.20 × 0.20 × 0.20 mm Theta range for data collection 3.58 to 28.27 deg. Limiting indices −23 <= h <= 29, −30 <= k <= 8, −21 <= l <= 37 Reflections collected/unique 6198/1586 [R(int) = 0.0289] Completeness to theta = 27.0 99.0% Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.7336and 0.7336 Refinement method Full-matrix least-squares on F² Data/restraints/parameters 1586/50/89 Goodness-of-fit on F² 1.072 Final R indices [I > R₁ = 0.0236, wR₂ = 0.0715 2sigma(I)] R indices (all data) R₁ = 0.0264, wR₂ = 0.0724 Largest diff. peak and hole 1.128 and −0.532 e. Å⁻³

TABLE 9 Crystal data and structure refinement. for compound Y-FTZBP-MOF (5) Identification code 5 Empirical formula C₈₄ H₄₂ F₆ N₂₄O₂₆Y₆ Formula weight 2450.88 Temperature 100(2) K Wavelength 1.54178 Å Crystal system, space group Cubic, Fm-3m Unit cell dimensions a = 29.447(3) Å alpha = 90 deg. b = 29.447(3) Å beta = 90 deg. c = 29.447(3) Å gamma = 90 deg. Volume 25535(4) Å³ Z, Calculated density 4, 0.638 Mg/m³ Absorption coefficient 2.074 mm⁻¹ F(000) 4840 Crystal size 0.20 × 0.20 × 0.20 mm Theta range for data collection 6.01 to 65.64 deg. Limiting indices −34 <= h <= 24, −34 <= k <= 34, −23 <= l <= 33 Reflections collected/unique 20849/1148 (R(int) = 0.0198] Completeness to theta = 65.64 98.3% Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.6818 and 0.6818 Refinement method Full-matrix least-squares on F² Data/restraints/parameters 1148/71/89 Goodness-of-fit on F² 1.059 Final R. indices [I > R₁ = 0.0337, wR₂ = 0.1010 2sigma(I)] R indices (all data) R₁ = 0.0340, wR₂ = 0.1016 Largest diff. peak and hole 0.675 and −0.283 e. Å⁻³

TABLE 10 Crystal data and structure refinement for compound Tb-FBPDC-MOF (6) Identification code 6 Empirical formula C₈₄ H₄₂ F₆ O₃₈ Tb₆ Formula weight 2726.70 Temperature 100(2) K Wavelength 1.54178 Å Crystal system, space group Cubic, Fm-3m Unit cell dimensions a = 27.5127(12) Å alpha = 90 deg. b = 27.5127(12) Å beta = 90 deg. c = 27.5127(12) Å gamma = 90 deg. Volume 20825.7(16) Å³ Z, Calculated density 4, 0.870 Mg/m³ Absorption coefficient 10.186 mm⁻¹ F(000) 5176 Crystal size 0.10 × 0.10 × 0.10 mm Theta range for data collection 2.78 to 63.44 deg. Limiting indices −26 <= h <= 31, −29 <= k <= 31, −26 <= l <= 31 Reflections collected/unique 20823/917 [R(int) = 0.0442] Completeness to theta = 63.44 99.6% Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.4290 and 0.4290 Refinement method Full-matrix least-squares on F² Data/restraints/parameters 917/38/59 Goodness-of-fit on F² 1.051 Final R indices [I > R₁ = 0.0402, wR₂ = 0.1105 2sigma(I)] R indices (all data) R₁ = 0.0420, wR₂ = 0.1125 Largest diff. peak and hole 0.839 and −0.716 e. Å⁻³

TABLE 11 Crystal data and structure refinement for compound Tb-DFBPDC-MOF (7) Identification code 7 Empirical formula C₈₄ H₃₆ F₁₂ O₄₆Tb₆ Formula weight 2962.65 Temperature 100(2) K Wavelength 1.54178 Å Crystal system, space group Cubic, Fm-3m Unit cell dimensions a = 27.4756(7) Å alpha = 90 deg. b = 27.4756(7) Å beta = 90 deg. c = 27.4756(7) Å gamma = 90 deg. Volume 20741.6(9) Å³ Z, Calculated density 4, 0.949 Mg/m³ Absorption coefficient 10.332 mm⁻¹ F(000) 5624 Crystal size 0.15 × 0.15 × 0.15 mm Theta range for data collection 4.55 to 67.93 deg. Limiting indices −23 <= h <= 31, −32 <= k <= 29, −28 <= l <= 31 Reflections collected/unique 5524/5524 [R(int) = 0.000] Completeness to theta = 66.60 98.7% Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.3063 and 0.3063 Refinement method Full-matrix least-squares on F² Data/restraints/parameters 5524/4/38 Goodness-of-fit on F² 1.051 Final R indices [I > R₁ = 0.0732, wR₂ = 0.2048 2sigma(I)] R indices (all data) R₁ = 0.0768, wR₂ = 0.2085 Largest diff. peak and hole 1.137 and −1.339 e. Å⁻³

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Other embodiments are within the scope of the following claims. 

What is claimed is:
 1. A composition, comprising: a molecular building block (MBB) having the formula [RE₆(μ₃-OH)₈(O₂C—)₁₂], wherein RE is a rare earth metal ion.
 2. The composition of claim 1, wherein the rare earth metal ion is selected from cerium, dysprosium, erbium, europium, gadolinium, holmium, lanthanum, lutetium, neodymium, praseodymium, promethium, samarium, scandium, terbium, thulium, ytterbium, yttrium, and combinations thereof.
 3. The composition of claim 1, further comprising a bidentate ligand.
 4. The composition of claim 3, wherein the bidentate ligand includes one or more of carboxylates, trizolates, tetrazolates, pyrazolates, and fluoros.
 5. The composition of claim 4, wherein the carboxylates are dicarboxylates.
 6. The composition of claim 4, wherein the trizolates are ditrizolates.
 7. The composition of claim 4, wherein the tetrazolates are ditetrazolates.
 8. The composition of claim 4, wherein the pyrazolates are dipyrazolates.
 9. The composition of claim 3, wherein the bidentate ligand is 2-fluoro-4-(1H-tetrazol-5-yl)benzoate (FTZB), 4-(1H-tetrazol-5-yl)benzoate (TZB), 3-fluoro-4′-(2H-tetrazol-5-yl)biphenyl-4-carboxylate (FTZBP), 3-fluorobiphenyl-4,4′-dicarboxylate (FBPDC), or 3,3′-difluorobiphenyl-4,4′-dicarboxylate (DFBPDC).
 10. A method of making a metal-organic framework (MOF) composition, comprising: contacting one or more molecular building blocks (MBBs) with one or more bidentate ligands, wherein the MBBs have the formula [RE₆(μ₃-OH)₈(O₂C—)₁₂], wherein RE is a rare earth metal ion.
 11. The method of claim 10, wherein the rare earth metal ion is selected from cerium, dysprosium, erbium, europium, gadolinium, holmium, lanthanum, lutetium, neodymium, praseodymium, promethium, samarium, scandium, terbium, thulium, ytterbium, yttrium, and combinations thereof. 